1. Field of the Invention
The present invention generally relates to an improved method and apparatus for metal melting, refining and processing, for example, steel making in an electric arc furnace (EAF), and more particularly, to an auxiliary burner or lance for the addition of thermal energy by the combustion of fuel, the injection of oxidizing gas for melt refining, foamy slag production or post combustion of carbon monoxide, and the injection of particulates for slag and foamy slag production.
2. Description of Background Art
Oxygen and carbon lances are known in the art of metal melting, refining and processing, particularly steel making, to be useful for the injection of carbon and oxygen for many steps in the process. In addition, auxiliary burners have been used to provide additional thermal energy and supersonic oxygen to these processes.
An electric arc furnace makes steel by first using an electric arc to melt one or more charges of scrap metal which is placed within the furnace. The scrap is charged by dumping it into the furnace through the roof from buckets which also could include charged carbon and slag forming materials. The arc melts the scrap into a molten pool of metal, an iron carbon melt, which accumulates at the bottom of the furnace. After a flat bath has been formed by melting of all the scrap introduced, the electric arc furnace enters a refining or decarburizing phase. In this phase, the metal continues to be heated by the arc until the slag forming materials combine with impurities in the melt and rise to the surface as slag. The charged carbon when the iron carbon melt reaches a boiling temperature combines with any oxygen present in the bath to form carbon monoxide bubbles which rise to the surface of the bath. Generally, at this time supersonic flows of oxygen are blown at the bath with from either lances or burners to produce a decarburization of the bath by the oxidation of the carbon contained in the bath. The carbon content of the bath is reduced to under 2% carbon where the iron carbon melt becomes steel and then further reduced until the grade of steel desired is produced, down to less than 0.2% for low carbon steels.
In higher power electric arc furnaces it is becoming a standard practice to use a long arc. When a long electric arc is struck, the radiative component of the arc above the bath can cause degradation to the furnace walls and the surrounding furnace equipment. The danger of this damage occurring is greatest after a significant portion of the scrap has been melted and the walls of the furnace are exposed to heat radiation from the arc and similarly thereafter during refining and flat bath conditions. Steelmakers have developed a foamy slag practice to protect the furnace components from the arc where the slag layer of impurities covering the iron carbon melt is foamed to increase its volume and rise above the arc. This foaming of the slag creates an insulative barrier between the arc and the furnace walls thereby protecting them from over heating. The overall process is also improved as the excess heat from the radiation of the arc is now used to heat the bath. Slag foaming practice differs for each EAF and operator preference. Slag foaming can begin as early as the first charge of scrap and can continue until the molten steel is tapped and the slag discarded.
The slag is conventionally foamed by bubbles of carbon monoxide caused from the oxidation of carbon with oxygen. To effectively foam slag these carbon monoxide bubbles should occur in the iron carbon melt, at the melt-slag interface or in the slag itself. If the slag has the right temperature, chemistry and viscosity, the carbon monoxide bubbles become entrained in the slag and cause its volume to increase rapidly to produce a lather or foam. Initially, the carbon which is charged with the scrap may form carbon monoxide bubbles by combining with oxygen which is in the bath or furnace during boiling or from additional oxygen blown into the bath with lances or burners. In addition, when the slag is hot, injected carbon particles either from lances or burners are used to form carbon monoxide bubbles by combining with FeO in the slag. The slag should be hot because the combination of carbon and FeO is an endothermic reaction which requires heat. Still further, oxygen may be present, either by injection from lances or burners or the furnace atmosphere, to oxidize carbon in the slag itself to carbon monoxide bubbles. In the latter case, the carbon in the slag may have been injected at the same time and from the same equipment as the carbon combining with the FeO to form carbon monoxide bubbles.
It is normal practice to inject the carbon and oxygen through burners or lances, or combinations thereof, at the same time, or nearly the same time, and directed to the same location, or nearly the same location. However, until the present invention, there have been some problems with directing the carbon and oxygen to the same, or nearly the same, location. The burners and lances previously used to provide this function were not very successful and efficient as they could be, because they direct the flows of carbon and oxygen in different directions.
Recently, there have been some attempts to combine oxygen and carbon injection lances with the oxy-fuel burner function. An important question for the integration of these functions into one apparatus has been whether to retain particulate injection capability or supersonic oxygen capability because both functions are the most advantageous if located along the central axis of the lance or burner.
Particulate injection is best done through a straight conduit which is located along the central axis of the apparatus used. A straight conduit is conventional because the particulates injected into a steel making furnace are highly abrasive and will wear out bends or other restrictions to their flow quickly. This is one of the reasons why particulates have not be injected through the same conduit as the one used for supersonic oxygen of a burner, the particulates would quickly wear out the converging restriction of the nozzle. A central conduit is preferred because it is highly disadvantageous to break the stream into more than one flow because one would like to concentrate particulates in a specific area. Also, the size of the particles and amount of particulates used for an injection is large in mass compared to other injected materials, such as gases, and a relatively large conduit is needed for reasonable flow rates.
Laval or supersonic nozzles are usually used in the production of high speed streams of oxidizing gas for injection into a steel making furnace. These supersonic gas flows are produced by the converging/diverging shape of the nozzle which at above a critical pressure causes the gas flow though the nozzle to become supersonic. Usually, an conduit is machined centrally in a lance or burner and then the passage is fitted with a converging/diverging section or nozzle. A large centrally located nozzle is desired because of the flow rates of supersonic oxygen desired.
It is also highly desirable to provide a subsonic flow of oxidizing gas for the burning of fuel, including regular fuel and carbon monoxide for post combustion, for the addition of auxiliary thermal energy, and the supersonic oxygen flow for providing oxygen in iron melt decarburization, assisting in foamy slag production or post combustion of carbon monoxide. A burner which provides subsonic and supersonic flows of oxygen through the same centrally located conduit is manufactured and commercially sold by Process Technology International, Inc. of Tucker, Georgia, the assignee of the present invention. The subsonic flow is produced by providing a pressure in the supply conduit lower than the critical pressure of the Laval nozzle being used in the conduit. When supersonic oxygen is needed, the pressure in the supply conduit is increased to above the critical pressure.
One attempt to combine the functions of carbon injection and supersonic oxygen in one apparatus is shown in U.S. Pat. No. 5,599,375. In FIGS. 3 and 5, a burner is described having carbon injection and supersonic oxygen. However, the carbon injection is not coaxial to the stream of oxidizing gas introduced through the burner and cannot be directed in sufficient quantities to be advantageous. Another attempt is shown in the same reference in FIG. 6 where a central carbon injection pipe is surrounded by a plurality of oxygen generating apertures which are described as Laval nozzles. This configuration is highly disadvantageous due to the small supersonic openings and dispersion of the supersonic oxygen due to flow turbulence of each small aperture interacting with that of the other apertures.
Therefore, there is a need for an apparatus which can direct a flow of particulate carbon and a flow of gaseous oxygen, preferably at a supersonic flow rate, in substantially the same direction and to the same location. There is also a need for a method whereby these flows can be introduced at the same time, or nearly the same time, such that an increased volume of carbon monoxide bubbles can be produced to foam slag in an electric arc furnace.
The invention provides an improved method and apparatus for metal melting, refining and processing, particularly steel making in an electric arc furnace.
In one embodiment of the method for a steel making process, auxiliary thermal energy is added to the process, particulates are introduced for the formation of slag, foamy slag or recarburization, and oxidizing gas is introduced for the decarburization of the melt, for the formation of foamy slag or for post combustion burning of carbon monoxide. Optionally, the oxidizing gas can be introduced as either a subsonic or supersonic flow.
According to the invention, one preferred embodiment of the apparatus comprises a burner or lance configuration which has a barrel shaped conduit for supplying a flow of particulates entrained in a carrier gas through its exit opening. Another barrel shaped conduit for oxidizing gas introduction is provided for producing a flow stream of oxidizing gas its exit opening. The exit openings of the particulate injection barrel and the oxidizing gas injection barrel are directed substantially in parallel to provide the flows to substantially the same location. More preferably, they are positioned side by side and are sized such that each diameter is approximately one half of the entrance diameter of the combustion chamber. According to another aspect of the invention, the oxygen injection barrel ends in an insert which contains a supersonic nozzle, preferably of the Laval type. The apparatus advantageously is capable of providing either subsonic or supersonic flows of oxidizing gas from the oxidizing gas conduit depending upon the oxidizing gas pressure.
The ends of the carbon injection conduit and the oxidizing gas injection conduit are mounted in a nozzle made of thermally conductive material, such as copper, to produce a nozzle assembly. The nozzle assembly has a conical surface which seats into a conical taper at the entrance end of a flame shaping chamber of a liquid cooled combustion chamber. The nozzle and the conduits disposed therein are thereby cooled by contact with the liquid cooled combustion chamber through the seat.
Another advantage of this arrangement is that the combustion chamber is shaped to provide a slight positive pressure for gases and/or particulates exiting the chamber. The positive pressure of the combustion chamber and the recessed position of the oxygen injection conduit and particulate injection conduit inside of the combustion chamber acts to prevent clogging and plugging when slag or steel splashing occurs in an electric arc furnace.
The burner includes a unique configuration which has a first barrel for providing finely pulverized particles entrained in a carrier gas and a second barrel for providing either supersonic or subsonic flows of an oxidizing gas. The exit ends of the first and second barrels are positioned side by side in a nozzle which inserts into the entrance of a flame shaping chamber of a fluid cooled combustion chamber. The nozzle also contains a plurality of fuel orifices for providing pressurized fuel to the combustion chamber and a plurality of secondary oxidizing gas orifices for providing a flow of an oxidizing gas around the periphery of the nozzle.
Because all of the flows of fuel, oxidizing gas and particulates pass through the flame shaping chamber, they are all substantially directed to the same location in the electric arc furnace. The directionality of the flows allows the burner to prepare a localized spot of the slag for foamy slag formation by initially heating the spot with thermal energy from the oxidation of the fuel by one or more of the oxidizing gases, from the oxidation of oxidizable components in the slag or the melt by the lancing of supersonic oxidizing gas, or from any combination of these. Once a localization in the slag is sufficiently prepared and heated, a flow of carbon is directed to the localized hot spot in the slag to reduce the FeO, and other oxides, in the slag to carbon monoxide and produce foamy slag. Optionally, the particulate carbon introduction can be accompanied by further oxidizing gas injection before, during or after the carbon injection.
Another advantage of an apparatus in this configuration is that it can be fixed in the furnace wall. Lances which have to be moved require relatively expensive lance manipulators and openings in the furnace through which they are positioned. If the opening is the slag door, the steel making process will be subject to the infiltration of ambient air through the slag door to produce another cold spot. The apparatus of the invention can be fixed in the furnace sidewall without air infiltration while providing a plurality of auxiliary functions highly desirable in steel making including melting and cutting scrap at cold spots, forming slag and foaming slag, decarburizing and recarburizing the melt, and post combustion of carbon monoxide.
These and other objects, aspects and features of the invention will be more clearly understood and better described when the following detailed description is read in conjunction with the attached drawings, wherein: